Tensile tests on a ultra high strength steel under in situ X raytomography made it possible to observe and to quantify 3Ddamage of this material. The analysis of the data gives accessto the main features of cavities (density and size) as well as localstrains and triaxiality within the whole tensile specimen. Basedon these results, a new damage model has been put forwardthat is noticeable by integrating cavity nucleation into the classicalRice and Tracey approach. The predictions of the newmodel fit significantly better with the experimental results.